Hey all, this is the best known way of spreading liquids the most efficient way.
The basic idea is to make a dotted L shape with water sources, and every single block within that area will become a water source block.
Guide Including Images
First of all you need to know how much area you want your water to cover.
Now you want to place a water source in every 2nd block on one end, represented by the orange blocks in the first screenshot.
Your water should look something like the second screenshot. Now you what you want to do is put a water source block in every 2nd block (represented by the orange blocks)
Your water should now look like the third screenshot. Congratulations you are now efficient with liquids!
This will work regardless of how big of an area you wish to cover.
It will also work for odd shapes such as this: